We evaluated the effects of competition for space between mixed turf algae (MTA) and the hermatypic coral Montastraea faveolata Ellis and Solander (1786) using reciprocal transplantation of 17 cm² cores and by measuring the response of some biological parameters of the coral: zooxanthellae density, mitotic index, chlorophyll a (Chl a) concentration, and tissue thickness. Mitotic index and Chl a zooxanthellae⁻¹ were not significantly affected by the competition, but zooxanthellae density, Chl a cm⁻², and tissue thickness were reduced in M. faveolata tissue surrounded by algae. Lower values have been reported for these three biological parameters of scleractinian corals subjected to stress conditions. Stressed M. faveolata surrounded by MTA were completely overgrown in 6–9 months. MTA frequently formed unattached cushions before the algae were attached to the coral skeleton. The cushions could be affecting M. faveolata by shading the coral tissue beneath the algae and probably causing stress to the tissue. Trapped sediments in the cushions may also be affecting M. faveolata by trapping sediments that cause smothering or burial of coral tissue. This is the first demonstration that algae directly stress a coral species and that MTA can be superior competitors than M. faveolata under experimental conditions. M. faveolata is sensitive to algae and bacteria, and the outlook for this coral species is poor if deleterious conditions act together in the Caribbean Sea.